This invention relates to instability detection and avoidance in a feedback system, in particular in a feedback active noise reduction system.
The presence of ambient acoustic noise in an environment can have a wide range of effects on human hearing. Some examples of ambient noise, such as engine noise in the cabin of a jet airliner, can cause minor annoyance to a passenger. Other examples of ambient noise, such as a jackhammer on a construction site can cause permanent hearing loss. Techniques for the reduction of ambient acoustic noise are an active area of research, providing benefits such as more pleasurable hearing experiences and avoidance of hearing losses.
Many conventional noise reduction systems utilize active noise reduction techniques to reduce the amount of noise that is perceived by a user. Active noise reduction systems are commonly implemented using feed-forward, feedback, or a combination of feed-forward and feedback approaches. Feedback based systems typically measure a noise sound wave, possibly combined with other sound waves, near an area where noise reduction is desired (e.g., in an acoustic cavity such as an ear cavity). In general, the measured signals are used to generate an “anti-noise signal” which is a phase inverted and scaled version of the measured noise. The anti-noise signal is provided to a noise cancellation driver which transduces the signal into a sound wave which is presented to the user. When the anti-noise sound wave produced by the noise cancellation driver combines in the acoustic cavity with the noise sound wave, the two sound waves cancel one another due to destructive interference. The result is a reduction in the noise level perceived by the user in the area where noise reduction is desired.
Feedback systems generally have the potential of being unstable and producing instability based distortion. For example, as understood based on classical analysis of feedback systems, if the gain of a feedback loop is greater than 1 at a frequency where the phase of the feedback loop is 180°, oscillatory additive signals can be generated at that frequency. Such a situation can also be described as the phase margin, which is the margin to reach 180° phase at a frequency at which the gain is 1, of the system being zero or negative.
In an acoustic active noise reduction system, at least a part of the feedback path can include an acoustic component. Although electrical or digital components of the feedback path can be directly controlled in an active noise reduction system, the acoustic component may be subject to variation, for example, as a result of variation in the physical characteristics of the acoustic path.